Pseudomonas aeruginosa is an opportunistic, ubiquitous gram-negative bacterium accounting for a large number of nosocomial infections in immunocompromised hosts. Additionally, it colonizes the lungs of patients suffering from cystic fibrosis, which can ultimately lead to lung failure. Based on the bacterium's high resistance towards antibiotics and the ability to form biofilms, these infections result in a high mortality of the patients. P. aeruginosa can switch to the biofilm mode of life, which serves as a physical barrier to survive antibiotic treatment and host immune defense. It is thereby able to maintain chronic infections. In a biofilm, bacteria are embedded in the extracellular matrix. This matrix is very complex and consists mainly of extracellular DNA, polysaccharides (pel, psi and alginate) and proteins. P. aeruginosa produces two soluble lectins under quorum-sensing control: LecA (or PA-IL) and LecB (or PA-IIL). These tetrameric carbohydrate binding proteins recognize specific monosaccharide residues in a calcium-dependent manner (C-type lectins). Whereas LecA is specific for D-galactose, LecB can bind L-fucosides and D-mannosides. These saccharide moieties are frequently found on the cell surface of the host and the bacterium and they are subunits of the polysaccharides of the extracellular matrix in the biofilm. Both lectins, LecA and LecB, were shown to be necessary for P. aeruginosa biofilm formation, suggesting a structural role for maintenance of the biofilm architecture. Since both lectins are virulence factors and necessary for biofilm formation, their inhibition is considered a promising approach for antipseudomonadal treatment. It has been shown that the inhibition of these lectins with multivalent carbohydrate-based ligands resulted in reduced biofilm formation of P. aeruginosa in vitro. Furthermore, inhalation of an aqueous galactose and fucose containing aerosol resulted in a reduction of respiratory tract infections with P. aeruginosa. 
The treatment of infections with P. aeruginosa has been a long-standing problem since antibiotics cannot reach all the bacteria which are embedded within the biofilm. Therefore, there is a need for therapeutic agents that can inhibit the formation of biofilm or disintegrate the biofilm.
LecB has an unusually high affinity for fucose residues. This has been explained by the crystal structure of the complex: two calcium ions in the binding site mediate the binding of one saccharide ligand to the protein.
Lewisa trisaccharide (α-Fuc1-4(βGal1-3)-GlcNAc) is the best known monovalent ligand of LecB with a Kd value of 210 nM and the crystal structure of the complex revealed an additional hydrogen bond of the GlcNAc-6-OH with the receptor. The remaining part of GlcNAc and the galactose moiety are, however, not in contact with the protein surface. Consequently, known synthetic inhibitors were simplified and based on the Fuc-α-1,4-GlcNAc disaccharide or on fucose alone. The anomeric centers of GlcNAc in Fuc-GlcNAc and of fucose were substituted with functional groups allowing facile conjugation to dendrimers, but also with small substituents to mimic the GlcNAc-6-OH in order to establish the known hydrogen bond with the receptor. This did, however, not lead to an increase in affinity of the resulting molecules when compared to the parent saccharide Lewisa: the disaccharide derivatives had lower dissociation constants than Lewisa and one monosaccharide derivative an equal dissociation constant to Lewisa. In another example, fucosylamides have been designed for binding to the fucose binding site and for establishing an additional hydrogen bond with LecB, but the affinity was threefold lower than methyl fucoside (Me-α-Fuc: Kd=430 nM).
The success of monovalent fucose-based ligands was limited. First, in all known LecB ligands, fucose glycoconjugates are α-linked and, consequently, all synthetic conjugates designed as inhibitors of LecB are terminal α-fucosides without modification of the terminal fucose. This may lead to unwanted, nonspecific binding of these molecules to various fucose-binding receptors in the host, e.g., the selectins, DC-SIGN and the mannose binding lectin. Second, the modifications introduced at the anomeric center failed to increase the binding affinity compared to the parent molecules methyl fucoside or Lewisa. Methyl and p-nitrophenyl β-fucosides have also been investigated and showed strongly reduced affinity for LecB as compared to their alpha-anomers.
LecB also binds mannosides via the 2, 3 and 4 hydroxyl groups that have the same relative orientation of the hydroxyl groups in fucose. Interestingly, the additional 6-OH of mannose is involved in a hydrogen bond to the side chain of Ser23 of LecB.
However, mannose lacks the lipophilic methyl group of fucose which establishes a contact with LecB in the crystal structure. This served as an explanation for the increased affinity of fucose over mannose (Me-α-Man: Kd=71 μM) towards LecB.
To date, a non-terminal glycoside, e.g. a modified mannose derivative, has not been known as an inhibitor of the glycoconjugate recognition of LecB. Such a non-terminal glycoside might lead to selective and highly potent P. aeruginosa lectin inhibitors, that do not block related lectins of the host's immune system. Moreover, small molecules are more desirable than dendrimers or polymers as orally-administered therapeutic agents.